1. Field of the Invention
The present invention relates to a grinding method for grinding a taper surface of parts or products and a grinding apparatus, and especially relates to a grinding method and a grinding apparatus suitable for mass-production.
2. Description of Related Art
Conventionally, methods for grinding a taper contact surface at a top end of a needle valve or a taper valve seat of a valve body, with which the contact surface makes contact, of a fuel injection valve for an internal combustion engine (hereinafter referred to as "engine") have been known, as disclosed in JP-A-60-242956, JP-A-3-3769 and JP-A-3-73258.
In the method disclosed in JP-A-60-242956, a lapping material is interposed between the seat surface and the contact surface, and the needle valve and the valve body are simultaneously ground by rubbing both. In the method disclosed in JP-A-3-3769, an abrasive material is interposed between a contact surface of a needle valve and a seat surface of a valve body, the needle valve and the valve body are rubbed while high-frequency vibration are applied thereto so that the contact surface of the needle valve and the seat surface of the valve body are ground.
In the grinding methods disclosed in JP-A-60-242956 and JP-A-3-3769, because the contact surface of the needle valve and the seat surface of the valve body are ground while each entire periphery thereof is bound, convex portions at each side are worn without being greatly affected by a vibration from an outside thereof. Therefore, circularity is improved, and sealing performance between the needle valve and the valve body improves. However, by grinding both with same taper angle, since contacting performance between the contact surface of the needle valve and the seat surface of the valve body improves excessively, there is a problem in that when the needle valve is opened to be separated from the valve seat timing is delayed or a groove in a peripheral direction is formed at a rubbed portion between the needle valve and the valve body to cause an abnormal shape of the atomized fuel.
To overcome such a problem, it is necessary to grind the needle valve and the valve body with grindstones having different angles, respectively. In the grinding method disclosed in 3-73258, to improve the circularity, there is employed a pressing and contacting type grinding method in which entire peripheral surfaces of a needle valve and a valve body are ground while being bound in contact with grindstones each having a concave-like or convex-like conical taper surface.
However, according to the grinding method disclosed in JP-3-73258, since each of the taper surfaces of the needle valve and the valve body needs to be ground while being in contact with grinding surface of the grinding stone, it is impossible to apply a reciprocating movement to the grindstone. Accordingly, grinding amount of the taper surface by means of the grindstone is extremely small; and therefore, there occurs a problem in that it is difficult to employ a pressing and contacting type grinding method as a finish-grinding process after performing a rough-grinding process, to improve the circularity. Further, to secure a desired circularity, since it is necessary to perform several tens of pressing and contacting type grinding processes; and therefore, there occurs a problem in that grinding time is prolonged and the manufacturing cost rises.
Next, grinding accuracy required for a grinding method of grinding a needle valve and a valve body separately will be described.
In view of the high consciousness for earth environmental protection, a required value of an amount of HC (Hydrocarbon emission) exhausted from a vehicle is strictly regulated year by year. The amount of HC contained in an exhaust gas increases by fuel leaked from a fuel injection valve into an engine; and therefore, it is required to reduce an amount of fuel leaked from the fuel injection valve. According to the U.S. emission regulation, since the required value of the amount of HC is changed from 0.125 g/mil. to 0.075 g/mil. as in 1997 with correspondence to the LEV (Low Emission Vehicle), it is necessary to limit fuel leakage amount to be equal to 0.8 mm.sup.3 /min. or less per a single fuel injection valve. That is, since an oil-tight accuracy five times as strict as the conventional one is required, it becomes more important to improve the accuracy of shape of seat portion of the fuel injection valve.
As factors having influence on an oil-tightness of the fuel injection valve, it turns out that there are surface roughness R, circularity Rns, and the difference of seat angles .DELTA..theta., which constitute the gap of a seat portion, between a needle valve 100 as a workpiece and a valve body 102 shown in FIGS. 9A and 9B. However, the difference of seat angles .DELTA..theta. cannot be changed from a predetermined value in view of the performance of the fuel injection valve. Resulting from studies of necessary accuracies of the roughness R and the circularity Rns, as a factor which does not satisfy the necessary accuracy for satisfying the required value of LEV, i.e., the fuel leakage amount is equal to 0.8 mm.sup.3 /min. or less, according to the conventional pressing and contacting type grinding method, it turns out that there is a circularity on a contact surface of the needle valve. To satisfy the condition where the fuel leakage amount is equal to 0.8 mm.sup.3 /min. or less, the circularity on the contact surface of the needle valve needs to be 0.3 .mu.m or less.
The conventional process for grinding a contact surface of the needle valve is composed of two processes including a rough-grinding process and a finish-grinding process performed after the rough-grinding process. To improve the circularity of the contact surface of the needle valve, there are conceived two methods. One method is to improve the circularity in the rough-grinding process, and the other is to increase a removed amount in the finish-grinding process according to the pressing and contacting type grinding method. However, it is difficult to employ the former method, because the cost increases to satisfy the condition where the circularity is equal to 0.3 .mu.m or less in the rough-grinding process according to the conventional method. In the conventional grinding method, there is a dispersion of the circularity in the rough-grinding process, in a range of approximately 1.5 .mu.m at the maximum. Accordingly, if the removed amount in the finish-grinding process according to the pressing and contacting type grinding method is equal to 1.5 .mu.m or more, it satisfies the condition where the circularity of the contact surface of the needle valve in the finish-grinding process is equal 0.3 .mu.m or less, which is the standard of the LEV.
FIG. 10 shows a conventional pressing and contacting type grinding method. A workpiece 100 is a needle valve, and a taper surface 101 is a contact surface of the needle valve. The workpiece 100 is pressed on a grindstone 104 by a constant force. By a rotation of the grindstone 104, the taper surface 101 of the workpiece 100 is ground. As shown in FIG. 11, the taper surface 101 is ground by the grindstone 104 such that abrasive grains 105 existing on a surface to be ground chip the taper surface 101. However, as shown in FIG. 12, a contact area (S) of the abrasive grain 105 with the taper surface 101 increases in accordance with an increase of a biting amount (t) of the abrasive grain having an obtuse cutting angle. As a result, as shown in FIG. 13, if the pressing force (F) is constant, a pressure (F/S) decreases in accordance with an increase of a biting amount (t) of the abrasive grain; and therefore, it becomes hard for the abrasive grain to bite the taper surface 101. That is, the abrasive grain passes on the same locus every lap; and therefore, after a predetermined time has elapsed, the abrasive grain 104 only rotates around the taper surface 101 without chipping the taper surface 101. To avoid this situation, a position of the abrasive grain 105 may be shifted from a groove 101a formed by the abrasive grain 105 so that the abrasive grain 105 can chip the taper surface 101 newly. However, in the grinding method for pressing the grindstone 104 on the workpiece 100, it is impossible to shift the abrasive grain 105 from the groove 101a to chip the taper surface 101 newly.
As a method for shifting the grinding locus of the abrasive grain while the grindstone 104 is pressed on the workpiece 100, the grindstone may be separated from the workpiece temporarily and be pressed on the workpiece again. However, to satisfy the condition where the removed amount is equal to 1.5 .mu.m or less, twenty times of processes for pressing and contacting the grindstone on the workpiece are needed, and a time period of approximately 30 seconds therefor is required. This time period is much longer than that in the conventional method and cannot be employed as a grinding method for mass-production.
The above description is made with reference to the improvement of the circularity of the needle valve; however, the valve body contacting the needle valve has the same problem.
The grinding method and the grinding apparatus according to the present invention can be employed for any valves other than the above-mentioned needle valve and the valve body of the fuel injection valve to improve a circularity and a fluid-tightness thereof.